Freezing Time Predictions for Different Final Product Temperatures

A modification to an empirical food freezing time prediction formula is proposed that allows the formula to be used for a range of final product temperatures. Over a large data set (275 runs) the percentage difference between experimentally measured times and predictions has a mean of 0.2% and a standard deviation of 6.8% for the improved formula compared to -2.4% and 8.5%, respectively, prior to modification. Ninety percent of the predicted freezing times by the new method were within ± 11% of the experimentally measured times and ± 9% of predictions by an accurate finite difference scheme. This performance compares favorably with other published freezing time prediction methods. The same type of modification for varying final product temperature may be suitable for other empirical formulae.     

A Simplified Model for Freezing Time Calculations in Foods

A model is proposed for freezing time calculations which combines Plank's equation with the unsteady heat transfer solutions for the cooling of a slab of constant properties through the addition of pre-cooling, change of phase and tempering periods. The change of thermal properties with the ice content is taken into account by proposing average values for the different periods. No adjustable parameters are used in developing the model. Results are compared for the case of beef freezing with those obtained numerically by using a heat transfer model with simultaneous change of phase and with experimental measurements showing good agreement.     

食品冷冻技术的研究进展

冷冻技术的发展异常迅速,在食品工业中的应用也越来越广泛,将其更好地应用于食品中成为当前研究者较为关注的课题。重点对冷冻过程中的理论研究及其在食品工业中的应用进行综述,并简述了近年来国内外食品冷冻技术的现状与发展趋势。     

3种速冻甘薯食品的加工工艺

主要介绍了出口速冻甘薯、冷冻甘薯片和速冻甘薯茎尖的生产工艺和操作要点，以期为生产厂家起到一定的参考作用。     

Structural Studies on Unpackaged Foods during Their Freezing and Storage

ABSTRACT:  During the freezing and frozen storage of unpackaged foods, their surface is exposed to mass transfer with the environment. Basically, ice sublimes and forms a dry, porous layer. This fact alters the sensory characteristics of the products and originates an important quality loss. In this work, a mathematical model was used to predict the thickness of the dehydrated layer, taking into account the influence of operating conditions and food characteristics. Based on the predictions of the numerical model, 2 regression equations were proposed to calculate the size of the dehydrated layer after freezing and storage, having the operating conditions and food properties as parameters. Besides, the frozen storage of the products was studied over different time periods (1, 2, and 3 mo) using beef cylinders and slices, as well as minced beef balls and hamburgers. The dimension of the dehydrated layer and the induced changes in the food surface structure were determined by image analysis and environmental scanning electron microscopy (ESEM). The experiments determined an increase in the depth of the dehydrated layer when storage times are longer, which could be adequately related to storage conditions through the predictions from the numerical model and the regression equations.     

Bacterial Extracellular Ice Nucleator Effects on Freezing of Foods

Extracellular ice nucleators (ECINs) were incorporated into foods and subjected to subzero freezing. Time-temperature profiles, ice-formation patterns and textures were examined by thermocouple, microscopy and texture analyzer. Onset temperatures (initial freezing), enthalpies and freezing rates were measured by DSC. Addition of ECINs to liquid foods elevated ice nucleation temperatures and promoted freezing. Solid or semisolid products frozen with ECINs resulted in a fiber-like texture. These effects were more apparent at –10°C or higher. Differential scanning calorimetry revealed onset temperatures were increased 11°C by addition of ECINs, but length of time to complete the phase transition was extended at constant cooling rates. Results indicated that ECINs can be used instead of whole bacterial cells for efficient freezing and textural modification.     

A Method to Determine Initial Freezing Point of Foods

An accurate initial freezing point T_sh is required for reliable analysis of a food freezing or thawing process. Two previous formulae for accurately estimating enthalpies, one for temperatures above T_sh and another below T_sh, were used for this development. The estimation of T_sh was based on the mathematical formulation of continuity in food enthalpies estimated by both formulae at T_sh. The method was validated by estimating T_sh values of aqueous solutions of sucrose and NaCl at different solutions. There was fair agreement in the T_sh values of fresh fruit and vegetables, lean beef meat and lean fish meat estimated by the proposed and a previous method. One potential advantage of our new method was the direct estimation of T_sh through analysis of food enthalpies collected at different temperatures.     

平板状食品冻结时间的数值预测

采用有限差分法获得平板状食品冻结时间的数值解。对凝固区（两相区）的参数进行处理,使食品在整个冻结过程可以选用统一的差分格式;通过凝固区释放热量的累积效果,判断凝固区界面的移动;对常见的平板状食品的冻结过程进行计算机模拟来预测冻结时间,并通过实验来验证可靠度。结果表明,这种计算方法可以应用在实际生产中。     

Frozen Foods Treated by Pressure Shift Freezing: Proteins and Enzymes

ABSTRACT: This work was focused on the changes that potatoes undergo after pressure shift freezing (PSF) treatment on protein content, peroxidase (POD), and polyphenoloxidase (PPO) enzymes. The samples were subjected to different conditions of P/T (210 MPa at −20°C; 180 MPa at −16°C; 150 MPa at −12°C; and 120 MPa at −8°C). Results showed that none of these conditions were enough to inactivate the POD and PPO enzymes. The POD was more resistant than PPO. Three isoenzymes of POD (61, 51, and 46 kDa) were detected. The protein content decreased about 50% after the PSF treatment at 210 MPa, −20°C; however, sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed 2 bands of about 21 kDa quite resistant to the treatment. On the contrary, mass spectrometry revealed among the resistant proteins, 2 fragments of pH 8.5 and about 20 kDa were more sensitive to 150 MPa, −12°C. The 2 fragments matched to a Solanum tuberosum proteinase inhibitor II protein     

Water Activity and Freezing Point Depression of Aqueous Solutions and Liquid Foods

Water activity (a_w) of binary aqueous solutions and liquid foods (skim milk, coffee beverages, peach and tomato juices) at different total solids concentration (by wt).was evaluated, measuring freezing point depression (FPD). The calculated values differed from those obtained by electric hygrometer by less than 0.01 a, units. Water activity was also evaluated in model systems containing ethanol, for which a_w determination by conventional electric hygrometer is not possible.     

Predictions of Microbial Thermal Inactivation in Solid Foods: Isothermal and Non-isothermal Conditions

This work focuses on the use of the Gompertz-inspired model to predict the thermal inactivation behaviour of microorganisms obtained in solid food products, validated for isothermal and non-isothermal conditions. Experiments were carried out in parsley, artificially inoculated with Listeria innocua. For the isothermal conditions tested, the predictive ability of the model was confined. The higher the temperature, the higher deviations observed (i.e. the model underestimates the inactivation behaviour). However, for the non-isothermal condition tested, the model predicted the microbial response accurately.     

软冷冻条件对动物性食物营养成分的影响

为评价软冷冻条件对动物性食物储存的影响，选择动物性食品为研究样品，观察在软冷冻（-7℃）和传统冷冻（-18℃）条件下，食物的外在表观变化和营养成分的变化。结果表明，在适宜的短期储存条件下，软冷冻可以达到人们对食物储存的保鲜要求，而且保持食物的营养素水平。该研究为软冷冻冰箱的开发和研究提供了科学依据。     

Freezing Time Modeling for Small Finite Cylindrical Shaped Foodstuff

A model was developed consisting of a modified Plank's equation to estimate phase change time and two unsteady state cross-product heat transfer equations for estimating precooling and tempering times. It accurately predicted total time to proceed from an initial temperature above the freezing point to a final temperature of ?18°C. A correction factor was developed and incorporated in the P term in Plank's equation to correct for the effect of initial and freezer medium temperatures and heat transfer coefficients. The model, tested over a broad range of freezing conditions, had a mean absolute error of 5.9% in predicted values relative to experimental values.     

Modeling Flow and Heat Transfer During Freezing of Foods in Forced Airstreams

ABSTRACT: Mathematical modeling of food freezing has been limited to the modeling of the internal heat transfer where the external convective heat-transfer coefficients are assumed or empirically estimated. Previous procedures followed to solve the external boundary layer in tandem with the internal heat transfer were constrained by numerical complexities due to the transient nature of the heat transfer, requiring unsteady formulation for the flow. In this article, attempts have been made to decouple the flow and heat transfer equations for the external boundary layer flow over a food product being frozen. The flow equations have been solved as a steady-state problem using Falker-Skan transformations of the boundary layer equation. The heat-transfer equation for fluid flow is solved as an unsteady-state problem in conjunction with the internal heat transfer and phase change inside the product undergoing freezing. The model is validated for a case of air-impingement freezing.     

Assessment of Freezing Time Prediction Methods

A systematic procedure for testing the accuracy of freezing time prediction methods is discussed. It involves carrying out freezing time predictions by an accurate numerical method as well as the method under test, and careful cross-checking of prediction errors for correlation with a number of physical parameters. Criteria are given for selection of an appropriate numerical method. The need for systematic cross-checking is illustrated by comparisons between four recent prediction methods. The comparisons show the strengths and weaknesses of the four methods. They also indicate general shortcomings in available methods and data, and so highlight those areas in which future research might be most profitable.     

Use of Average Molecular Weights for Product Categories to Predict Freezing Characteristics of Foods

ABSTRACT:  In the design of food freezing process, food property parameters, initial freezing temperature ( T_Fi ), and frozen water fraction ( X_I ) are required. The predictive approaches of these 2 parameters have been developed based on mass fractions and molecular weights of specific food components such as proteins, carbohydrates, minerals, and acids/bases. In this study, the molecular weights of the key mineral and acid/base components were successfully represented using average molecular weights (     ) and 4  T_Fi  and  X_I  calculation approaches were proposed. Based on an analysis of 212 food products, the absolute differences (AD) between the experimental and predicted  T_Fi  values for the 4 approaches were small. The prediction for the food model category was excellent with average AD (     ) values as low as ± 0.03 °C. For the other food categories, the prediction efficiency was impressive with     values between ± 0.22 and ± 0.38 °C. The predicted relationship between temperature and  X_I  for all analyzed food products provided close agreements with experimental data.     

简单形状食品冻结时间计算方法的分析研究

冻结时间对食品的冻结质量和能耗有重要意义.通过对几种简单形状食品冻结时间计算模型和相应公式的分析和计算,对它们在特定工况下应用的准确性和可靠性进行了比较,并对相关计算参数的选取给出了说明和建议.     

响应面法优化食品浸渍速冻冻结液配方

为优化食品浸渍快速冻结的冻结液配方,以甜菜碱、丙二醇、氯化钠为主要组分,添加少量甘氨酸、低聚木糖、甘露醇,组成低温冻结液,在单因素试验基础上,运用Box-Behnken试验设计原理进行试验设计,探讨浸渍快速冻结的冻结液配方的最佳配比。结果表明：以占冻结液总质量分数计,由21%甜菜碱、15%丙二醇、10%氯化钠、1%甘氨酸、1%低聚木糖、1%甘露醇和水组成的多元冻结液,冻结点可达－38.10℃,可用作食品浸渍快速冻结的冻结液,用于水产品及冷冻方便食品的冷冻加工。     

A Simplified Analytical Model for Thawing Time Calculation in Foods

A simplified analytical model for the thawing time prediction of simple-shaped foodstuffs was developed. It was assumed in the model that the solution to the unsteady state, unidirectional heat conduction equation with constant thermophysical properties was valid throughout the thawing operation. The latent heat effects were incorporated into effective diffusivity terms defined for the various phases of the thawing operation. The predictions of the model were compared to experimental and numerical data obtained on thawing of infinite slabs, infinite cylinders, and spheres. The level of agreement between the predictions of the present model, and the experimental and numerical data was quite satisfactory.     

果蔬汁等液态食品之冻结点的经验计算

冻结点是果蔬汁等液态食品冷冻加工与保藏中的重要参数，冻结点和冻结点的下降与果蔬汁等液态食品的组成分及浓度有关。本文介绍了几种通过计算冻结点下降值求得冻结点的经验公式。